xref: /freebsd/sys/dev/usb/gadget/g_audio.c (revision eb69d1f144a6fcc765d1b9d44a5ae8082353e70b)
1 /*-
2  * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
3  *
4  * Copyright (c) 2010 Hans Petter Selasky. All rights reserved.
5  *
6  * Redistribution and use in source and binary forms, with or without
7  * modification, are permitted provided that the following conditions
8  * are met:
9  * 1. Redistributions of source code must retain the above copyright
10  *    notice, this list of conditions and the following disclaimer.
11  * 2. Redistributions in binary form must reproduce the above copyright
12  *    notice, this list of conditions and the following disclaimer in the
13  *    documentation and/or other materials provided with the distribution.
14  *
15  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
16  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
17  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
18  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
19  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
20  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
21  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
22  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
23  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
24  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
25  * SUCH DAMAGE.
26  */
27 
28 /*
29  * USB audio specs: http://www.usb.org/developers/devclass_docs/audio10.pdf
30  *		    http://www.usb.org/developers/devclass_docs/frmts10.pdf
31  *		    http://www.usb.org/developers/devclass_docs/termt10.pdf
32  */
33 
34 #include <sys/param.h>
35 __FBSDID("$FreeBSD$");
36 
37 #include <sys/stdint.h>
38 #include <sys/stddef.h>
39 #include <sys/queue.h>
40 #include <sys/systm.h>
41 #include <sys/kernel.h>
42 #include <sys/bus.h>
43 #include <sys/linker_set.h>
44 #include <sys/module.h>
45 #include <sys/lock.h>
46 #include <sys/mutex.h>
47 #include <sys/condvar.h>
48 #include <sys/sysctl.h>
49 #include <sys/sx.h>
50 #include <sys/unistd.h>
51 #include <sys/callout.h>
52 #include <sys/malloc.h>
53 #include <sys/priv.h>
54 
55 #include <dev/usb/usb.h>
56 #include <dev/usb/usb_cdc.h>
57 #include <dev/usb/usbdi.h>
58 #include <dev/usb/usbdi_util.h>
59 #include <dev/usb/usbhid.h>
60 #include "usb_if.h"
61 
62 #define	USB_DEBUG_VAR g_audio_debug
63 #include <dev/usb/usb_debug.h>
64 
65 #include <dev/usb/gadget/g_audio.h>
66 
67 enum {
68 	G_AUDIO_ISOC0_RD,
69 	G_AUDIO_ISOC1_RD,
70 	G_AUDIO_ISOC0_WR,
71 	G_AUDIO_ISOC1_WR,
72 	G_AUDIO_N_TRANSFER,
73 };
74 
75 struct g_audio_softc {
76 	struct mtx sc_mtx;
77 	struct usb_callout sc_callout;
78 	struct usb_callout sc_watchdog;
79 	struct usb_xfer *sc_xfer[G_AUDIO_N_TRANSFER];
80 
81 	int	sc_mode;
82 	int	sc_pattern_len;
83 	int	sc_throughput;
84 	int	sc_tx_interval;
85 	int	sc_state;
86 	int	sc_noise_rem;
87 
88 	int8_t	sc_pattern[G_AUDIO_MAX_STRLEN];
89 
90 	uint16_t sc_data_len[2][G_AUDIO_FRAMES];
91 
92 	int16_t	sc_data_buf[2][G_AUDIO_BUFSIZE / 2];
93 
94 	uint8_t	sc_volume_setting[32];
95 	uint8_t	sc_volume_limit[32];
96 	uint8_t	sc_sample_rate[32];
97 };
98 
99 static SYSCTL_NODE(_hw_usb, OID_AUTO, g_audio, CTLFLAG_RW, 0, "USB audio gadget");
100 
101 #ifdef USB_DEBUG
102 static int g_audio_debug = 0;
103 
104 SYSCTL_INT(_hw_usb_g_audio, OID_AUTO, debug, CTLFLAG_RWTUN,
105     &g_audio_debug, 0, "Debug level");
106 #endif
107 
108 static int g_audio_mode = 0;
109 
110 SYSCTL_INT(_hw_usb_g_audio, OID_AUTO, mode, CTLFLAG_RWTUN,
111     &g_audio_mode, 0, "Mode selection");
112 
113 static int g_audio_pattern_interval = 1000;
114 
115 SYSCTL_INT(_hw_usb_g_audio, OID_AUTO, pattern_interval, CTLFLAG_RWTUN,
116     &g_audio_pattern_interval, 0, "Pattern interval in milliseconds");
117 
118 static char g_audio_pattern_data[G_AUDIO_MAX_STRLEN];
119 
120 SYSCTL_STRING(_hw_usb_g_audio, OID_AUTO, pattern, CTLFLAG_RW,
121     &g_audio_pattern_data, sizeof(g_audio_pattern_data), "Data pattern");
122 
123 static int g_audio_throughput;
124 
125 SYSCTL_INT(_hw_usb_g_audio, OID_AUTO, throughput, CTLFLAG_RD,
126     &g_audio_throughput, sizeof(g_audio_throughput), "Throughput in bytes per second");
127 
128 static device_probe_t g_audio_probe;
129 static device_attach_t g_audio_attach;
130 static device_detach_t g_audio_detach;
131 static usb_handle_request_t g_audio_handle_request;
132 
133 static usb_callback_t g_audio_isoc_read_callback;
134 static usb_callback_t g_audio_isoc_write_callback;
135 
136 static devclass_t g_audio_devclass;
137 
138 static void g_audio_watchdog(void *arg);
139 static void g_audio_timeout(void *arg);
140 
141 static device_method_t g_audio_methods[] = {
142 	/* USB interface */
143 	DEVMETHOD(usb_handle_request, g_audio_handle_request),
144 
145 	/* Device interface */
146 	DEVMETHOD(device_probe, g_audio_probe),
147 	DEVMETHOD(device_attach, g_audio_attach),
148 	DEVMETHOD(device_detach, g_audio_detach),
149 
150 	DEVMETHOD_END
151 };
152 
153 static driver_t g_audio_driver = {
154 	.name = "g_audio",
155 	.methods = g_audio_methods,
156 	.size = sizeof(struct g_audio_softc),
157 };
158 
159 DRIVER_MODULE(g_audio, uhub, g_audio_driver, g_audio_devclass, 0, 0);
160 MODULE_DEPEND(g_audio, usb, 1, 1, 1);
161 
162 static const struct usb_config g_audio_config[G_AUDIO_N_TRANSFER] = {
163 
164 	[G_AUDIO_ISOC0_RD] = {
165 		.type = UE_ISOCHRONOUS,
166 		.endpoint = UE_ADDR_ANY,
167 		.direction = UE_DIR_RX,
168 		.flags = {.ext_buffer = 1,.pipe_bof = 1,.short_xfer_ok = 1,},
169 		.bufsize = G_AUDIO_BUFSIZE,
170 		.callback = &g_audio_isoc_read_callback,
171 		.frames = G_AUDIO_FRAMES,
172 		.usb_mode = USB_MODE_DEVICE,
173 		.if_index = 1,
174 	},
175 
176 	[G_AUDIO_ISOC1_RD] = {
177 		.type = UE_ISOCHRONOUS,
178 		.endpoint = UE_ADDR_ANY,
179 		.direction = UE_DIR_RX,
180 		.flags = {.ext_buffer = 1,.pipe_bof = 1,.short_xfer_ok = 1,},
181 		.bufsize = G_AUDIO_BUFSIZE,
182 		.callback = &g_audio_isoc_read_callback,
183 		.frames = G_AUDIO_FRAMES,
184 		.usb_mode = USB_MODE_DEVICE,
185 		.if_index = 1,
186 	},
187 
188 	[G_AUDIO_ISOC0_WR] = {
189 		.type = UE_ISOCHRONOUS,
190 		.endpoint = UE_ADDR_ANY,
191 		.direction = UE_DIR_TX,
192 		.flags = {.ext_buffer = 1,.pipe_bof = 1,},
193 		.bufsize = G_AUDIO_BUFSIZE,
194 		.callback = &g_audio_isoc_write_callback,
195 		.frames = G_AUDIO_FRAMES,
196 		.usb_mode = USB_MODE_DEVICE,
197 		.if_index = 2,
198 	},
199 
200 	[G_AUDIO_ISOC1_WR] = {
201 		.type = UE_ISOCHRONOUS,
202 		.endpoint = UE_ADDR_ANY,
203 		.direction = UE_DIR_TX,
204 		.flags = {.ext_buffer = 1,.pipe_bof = 1,},
205 		.bufsize = G_AUDIO_BUFSIZE,
206 		.callback = &g_audio_isoc_write_callback,
207 		.frames = G_AUDIO_FRAMES,
208 		.usb_mode = USB_MODE_DEVICE,
209 		.if_index = 2,
210 	},
211 };
212 
213 static void
214 g_audio_timeout_reset(struct g_audio_softc *sc)
215 {
216 	int i = g_audio_pattern_interval;
217 
218 	sc->sc_tx_interval = i;
219 
220 	if (i <= 0)
221 		i = 1;
222 	else if (i > 1023)
223 		i = 1023;
224 
225 	i = USB_MS_TO_TICKS(i);
226 
227 	usb_callout_reset(&sc->sc_callout, i, &g_audio_timeout, sc);
228 }
229 
230 static void
231 g_audio_timeout(void *arg)
232 {
233 	struct g_audio_softc *sc = arg;
234 
235 	sc->sc_mode = g_audio_mode;
236 
237 	memcpy(sc->sc_pattern, g_audio_pattern_data, sizeof(sc->sc_pattern));
238 
239 	sc->sc_pattern[G_AUDIO_MAX_STRLEN - 1] = 0;
240 
241 	sc->sc_pattern_len = strlen(sc->sc_pattern);
242 
243 	if (sc->sc_mode != G_AUDIO_MODE_LOOP) {
244 		usbd_transfer_start(sc->sc_xfer[G_AUDIO_ISOC0_WR]);
245 		usbd_transfer_start(sc->sc_xfer[G_AUDIO_ISOC1_WR]);
246 	}
247 	g_audio_timeout_reset(sc);
248 }
249 
250 static void
251 g_audio_watchdog_reset(struct g_audio_softc *sc)
252 {
253 	usb_callout_reset(&sc->sc_watchdog, hz, &g_audio_watchdog, sc);
254 }
255 
256 static void
257 g_audio_watchdog(void *arg)
258 {
259 	struct g_audio_softc *sc = arg;
260 	int i;
261 
262 	i = sc->sc_throughput;
263 
264 	sc->sc_throughput = 0;
265 
266 	g_audio_throughput = i;
267 
268 	g_audio_watchdog_reset(sc);
269 }
270 
271 static int
272 g_audio_probe(device_t dev)
273 {
274 	struct usb_attach_arg *uaa = device_get_ivars(dev);
275 
276 	DPRINTFN(11, "\n");
277 
278 	if (uaa->usb_mode != USB_MODE_DEVICE)
279 		return (ENXIO);
280 
281 	if ((uaa->info.bInterfaceClass == UICLASS_AUDIO) &&
282 	    (uaa->info.bInterfaceSubClass == UISUBCLASS_AUDIOCONTROL))
283 		return (0);
284 
285 	return (ENXIO);
286 }
287 
288 static int
289 g_audio_attach(device_t dev)
290 {
291 	struct g_audio_softc *sc = device_get_softc(dev);
292 	struct usb_attach_arg *uaa = device_get_ivars(dev);
293 	int error;
294 	int i;
295 	uint8_t iface_index[3];
296 
297 	DPRINTFN(11, "\n");
298 
299 	device_set_usb_desc(dev);
300 
301 	mtx_init(&sc->sc_mtx, "g_audio", NULL, MTX_DEF);
302 
303 	usb_callout_init_mtx(&sc->sc_callout, &sc->sc_mtx, 0);
304 	usb_callout_init_mtx(&sc->sc_watchdog, &sc->sc_mtx, 0);
305 
306 	sc->sc_mode = G_AUDIO_MODE_SILENT;
307 
308 	sc->sc_noise_rem = 1;
309 
310 	for (i = 0; i != G_AUDIO_FRAMES; i++) {
311 		sc->sc_data_len[0][i] = G_AUDIO_BUFSIZE / G_AUDIO_FRAMES;
312 		sc->sc_data_len[1][i] = G_AUDIO_BUFSIZE / G_AUDIO_FRAMES;
313 	}
314 
315 	iface_index[0] = uaa->info.bIfaceIndex;
316 	iface_index[1] = uaa->info.bIfaceIndex + 1;
317 	iface_index[2] = uaa->info.bIfaceIndex + 2;
318 
319 	error = usbd_set_alt_interface_index(uaa->device, iface_index[1], 1);
320 	if (error) {
321 		DPRINTF("alt iface setting error=%s\n", usbd_errstr(error));
322 		goto detach;
323 	}
324 	error = usbd_set_alt_interface_index(uaa->device, iface_index[2], 1);
325 	if (error) {
326 		DPRINTF("alt iface setting error=%s\n", usbd_errstr(error));
327 		goto detach;
328 	}
329 	error = usbd_transfer_setup(uaa->device,
330 	    iface_index, sc->sc_xfer, g_audio_config,
331 	    G_AUDIO_N_TRANSFER, sc, &sc->sc_mtx);
332 
333 	if (error) {
334 		DPRINTF("error=%s\n", usbd_errstr(error));
335 		goto detach;
336 	}
337 	usbd_set_parent_iface(uaa->device, iface_index[1], iface_index[0]);
338 	usbd_set_parent_iface(uaa->device, iface_index[2], iface_index[0]);
339 
340 	mtx_lock(&sc->sc_mtx);
341 
342 	usbd_transfer_start(sc->sc_xfer[G_AUDIO_ISOC0_RD]);
343 	usbd_transfer_start(sc->sc_xfer[G_AUDIO_ISOC1_RD]);
344 
345 	usbd_transfer_start(sc->sc_xfer[G_AUDIO_ISOC0_WR]);
346 	usbd_transfer_start(sc->sc_xfer[G_AUDIO_ISOC1_WR]);
347 
348 	g_audio_timeout_reset(sc);
349 
350 	g_audio_watchdog_reset(sc);
351 
352 	mtx_unlock(&sc->sc_mtx);
353 
354 	return (0);			/* success */
355 
356 detach:
357 	g_audio_detach(dev);
358 
359 	return (ENXIO);			/* error */
360 }
361 
362 static int
363 g_audio_detach(device_t dev)
364 {
365 	struct g_audio_softc *sc = device_get_softc(dev);
366 
367 	DPRINTF("\n");
368 
369 	mtx_lock(&sc->sc_mtx);
370 	usb_callout_stop(&sc->sc_callout);
371 	usb_callout_stop(&sc->sc_watchdog);
372 	mtx_unlock(&sc->sc_mtx);
373 
374 	usbd_transfer_unsetup(sc->sc_xfer, G_AUDIO_N_TRANSFER);
375 
376 	usb_callout_drain(&sc->sc_callout);
377 	usb_callout_drain(&sc->sc_watchdog);
378 
379 	mtx_destroy(&sc->sc_mtx);
380 
381 	return (0);
382 }
383 
384 
385 static int32_t
386 g_noise(struct g_audio_softc *sc)
387 {
388 	uint32_t temp;
389 	const uint32_t prime = 0xFFFF1D;
390 
391 	if (sc->sc_noise_rem & 1) {
392 		sc->sc_noise_rem += prime;
393 	}
394 	sc->sc_noise_rem /= 2;
395 
396 	temp = sc->sc_noise_rem;
397 
398 	/* unsigned to signed conversion */
399 
400 	temp ^= 0x800000;
401 	if (temp & 0x800000) {
402 		temp |= (-0x800000);
403 	}
404 	return temp;
405 }
406 
407 static void
408 g_audio_make_samples(struct g_audio_softc *sc, int16_t *ptr, int samples)
409 {
410 	int i;
411 	int j;
412 
413 	for (i = 0; i != samples; i++) {
414 
415 		j = g_noise(sc);
416 
417 		if ((sc->sc_state < 0) || (sc->sc_state >= sc->sc_pattern_len))
418 			sc->sc_state = 0;
419 
420 		if (sc->sc_pattern_len != 0) {
421 			j = (j * sc->sc_pattern[sc->sc_state]) >> 16;
422 			sc->sc_state++;
423 		}
424 		*ptr++ = j / 256;
425 		*ptr++ = j / 256;
426 	}
427 }
428 
429 static void
430 g_audio_isoc_write_callback(struct usb_xfer *xfer, usb_error_t error)
431 {
432 	struct g_audio_softc *sc = usbd_xfer_softc(xfer);
433 	int actlen;
434 	int aframes;
435 	int nr = (xfer == sc->sc_xfer[G_AUDIO_ISOC0_WR]) ? 0 : 1;
436 	int16_t *ptr;
437 	int i;
438 
439 	usbd_xfer_status(xfer, &actlen, NULL, &aframes, NULL);
440 
441 	DPRINTF("st=%d aframes=%d actlen=%d bytes\n",
442 	    USB_GET_STATE(xfer), aframes, actlen);
443 
444 	switch (USB_GET_STATE(xfer)) {
445 	case USB_ST_TRANSFERRED:
446 
447 		sc->sc_throughput += actlen;
448 
449 		if (sc->sc_mode == G_AUDIO_MODE_LOOP)
450 			break;		/* sync with RX */
451 
452 	case USB_ST_SETUP:
453 tr_setup:
454 
455 		ptr = sc->sc_data_buf[nr];
456 
457 		if (sc->sc_mode == G_AUDIO_MODE_PATTERN) {
458 
459 			for (i = 0; i != G_AUDIO_FRAMES; i++) {
460 
461 				usbd_xfer_set_frame_data(xfer, i, ptr, sc->sc_data_len[nr][i]);
462 
463 				g_audio_make_samples(sc, ptr, (G_AUDIO_BUFSIZE / G_AUDIO_FRAMES) / 2);
464 
465 				ptr += (G_AUDIO_BUFSIZE / G_AUDIO_FRAMES) / 2;
466 			}
467 		} else if (sc->sc_mode == G_AUDIO_MODE_LOOP) {
468 
469 			for (i = 0; i != G_AUDIO_FRAMES; i++) {
470 
471 				usbd_xfer_set_frame_data(xfer, i, ptr, sc->sc_data_len[nr][i] & ~3);
472 
473 				g_audio_make_samples(sc, ptr, sc->sc_data_len[nr][i] / 4);
474 
475 				ptr += (G_AUDIO_BUFSIZE / G_AUDIO_FRAMES) / 2;
476 			}
477 		}
478 		break;
479 
480 	default:			/* Error */
481 		DPRINTF("error=%s\n", usbd_errstr(error));
482 
483 		if (error != USB_ERR_CANCELLED) {
484 			/* try to clear stall first */
485 			usbd_xfer_set_stall(xfer);
486 			goto tr_setup;
487 		}
488 		break;
489 	}
490 }
491 
492 static void
493 g_audio_isoc_read_callback(struct usb_xfer *xfer, usb_error_t error)
494 {
495 	struct g_audio_softc *sc = usbd_xfer_softc(xfer);
496 	int actlen;
497 	int aframes;
498 	int nr = (xfer == sc->sc_xfer[G_AUDIO_ISOC0_RD]) ? 0 : 1;
499 	int16_t *ptr;
500 	int i;
501 
502 	usbd_xfer_status(xfer, &actlen, NULL, &aframes, NULL);
503 
504 	DPRINTF("st=%d aframes=%d actlen=%d bytes\n",
505 	    USB_GET_STATE(xfer), aframes, actlen);
506 
507 	switch (USB_GET_STATE(xfer)) {
508 	case USB_ST_TRANSFERRED:
509 
510 		sc->sc_throughput += actlen;
511 
512 		for (i = 0; i != G_AUDIO_FRAMES; i++) {
513 			sc->sc_data_len[nr][i] = usbd_xfer_frame_len(xfer, i);
514 		}
515 
516 		usbd_transfer_start(sc->sc_xfer[G_AUDIO_ISOC0_WR]);
517 		usbd_transfer_start(sc->sc_xfer[G_AUDIO_ISOC1_WR]);
518 
519 		break;
520 
521 	case USB_ST_SETUP:
522 tr_setup:
523 		ptr = sc->sc_data_buf[nr];
524 
525 		for (i = 0; i != G_AUDIO_FRAMES; i++) {
526 
527 			usbd_xfer_set_frame_data(xfer, i, ptr,
528 			    G_AUDIO_BUFSIZE / G_AUDIO_FRAMES);
529 
530 			ptr += (G_AUDIO_BUFSIZE / G_AUDIO_FRAMES) / 2;
531 		}
532 
533 		usbd_transfer_submit(xfer);
534 		break;
535 
536 	default:			/* Error */
537 		DPRINTF("error=%s\n", usbd_errstr(error));
538 
539 		if (error != USB_ERR_CANCELLED) {
540 			/* try to clear stall first */
541 			usbd_xfer_set_stall(xfer);
542 			goto tr_setup;
543 		}
544 		break;
545 	}
546 }
547 
548 
549 static int
550 g_audio_handle_request(device_t dev,
551     const void *preq, void **pptr, uint16_t *plen,
552     uint16_t offset, uint8_t *pstate)
553 {
554 	struct g_audio_softc *sc = device_get_softc(dev);
555 	const struct usb_device_request *req = preq;
556 	uint8_t is_complete = *pstate;
557 
558 	if (!is_complete) {
559 		if ((req->bmRequestType == UT_READ_CLASS_INTERFACE) &&
560 		    (req->bRequest == 0x82 /* get min */ )) {
561 
562 			if (offset == 0) {
563 				USETW(sc->sc_volume_limit, 0);
564 				*plen = 2;
565 				*pptr = &sc->sc_volume_limit;
566 			} else {
567 				*plen = 0;
568 			}
569 			return (0);
570 		} else if ((req->bmRequestType == UT_READ_CLASS_INTERFACE) &&
571 		    (req->bRequest == 0x83 /* get max */ )) {
572 
573 			if (offset == 0) {
574 				USETW(sc->sc_volume_limit, 0x2000);
575 				*plen = 2;
576 				*pptr = &sc->sc_volume_limit;
577 			} else {
578 				*plen = 0;
579 			}
580 			return (0);
581 		} else if ((req->bmRequestType == UT_READ_CLASS_INTERFACE) &&
582 		    (req->bRequest == 0x84 /* get residue */ )) {
583 
584 			if (offset == 0) {
585 				USETW(sc->sc_volume_limit, 1);
586 				*plen = 2;
587 				*pptr = &sc->sc_volume_limit;
588 			} else {
589 				*plen = 0;
590 			}
591 			return (0);
592 		} else if ((req->bmRequestType == UT_READ_CLASS_INTERFACE) &&
593 		    (req->bRequest == 0x81 /* get value */ )) {
594 
595 			if (offset == 0) {
596 				USETW(sc->sc_volume_setting, 0x2000);
597 				*plen = sizeof(sc->sc_volume_setting);
598 				*pptr = &sc->sc_volume_setting;
599 			} else {
600 				*plen = 0;
601 			}
602 			return (0);
603 		} else if ((req->bmRequestType == UT_WRITE_CLASS_INTERFACE) &&
604 		    (req->bRequest == 0x01 /* set value */ )) {
605 
606 			if (offset == 0) {
607 				*plen = sizeof(sc->sc_volume_setting);
608 				*pptr = &sc->sc_volume_setting;
609 			} else {
610 				*plen = 0;
611 			}
612 			return (0);
613 		} else if ((req->bmRequestType == UT_WRITE_CLASS_ENDPOINT) &&
614 		    (req->bRequest == 0x01 /* set value */ )) {
615 
616 			if (offset == 0) {
617 				*plen = sizeof(sc->sc_sample_rate);
618 				*pptr = &sc->sc_sample_rate;
619 			} else {
620 				*plen = 0;
621 			}
622 			return (0);
623 		}
624 	}
625 	return (ENXIO);			/* use builtin handler */
626 }
627